The long-range objectives of this project are to develop practicable procedures for classification of spotted fever- and typhus-group rickettsiae and to determine the nature and biological properties of rickettsial antigens and constitutents. Current specific interests are: 1. serotyping rickettsiae by microimmunofluorescence, 2. relationship of rickettsiae by means of toxin neutralization and polyacrylamide gel electrophoresis assays, and 3. nature and properties of rickettsial antigens determined by other specialized techniques.